1. Field of the Invention
This invention relates to a method for manufacturing a semiconductor device having a non-volatile memory cell region and a logic region including MOS transistors.
2. Description of the Related Art
A conventional method for manufacturing a semiconductor device having a non-volatile memory cell region and a logic region including MOS transistors will now be explained.
First, to form a first insulating film and a first polysilicon film in the memory cell region, a gate oxide film is formed over a semiconductor wafer or substrate by thermal oxidization, and then a polysilicon film is deposited on the oxide film.
Subsequently, only those portions of the gate oxide film and polysilicon film which are located in the logic region are removed from the wafer. Then, another insulating film is formed all over the wafer by thermal oxidization. A resist film is directly coated on the insulating film. Thereafter, an opening is formed in a desired portion of the resist film, through which impurity ions are implanted into the substrate of the logic region, thereby forming the channel region of a MOS transistor. The resist film is then removed, and a second polysilicon film is deposited over the memory cell region and logic region.
Thereafter, predetermined portions of the memory cell region and logic region are subjected to self-align dry etching, thus forming a memory cell and MOS transistors constituting a logic circuit, respectively.
However, this method has the following disadvantages:
1. Due to the process of implanting impurity ions into the logic region through the gate oxide film, the gate film is charged with part of the impurity ions, and traps are formed therein. Thus, the gate threshold voltage Vth of the MOSFET is unstable.
2. To implant impurity ions into the channel region of the logic region, the resist film is directly formed on a poly-poly insulating film (an insulating film formed between a floating gate and a control gate) in the non-volatile memory cell region, and on the gate oxide film in the logic region. This causes part of the resist film to remain as a pollutant in the poly-poly insulating film and in the gate oxide film.